In a spark discharge emission analyzer, an amount of energy stored in a capacitor is supplied to a discharge electrode to generate a spark discharge between the electrode and a metallic sample, whereby the atoms of the elements contained in the metallic sample are vaporized, and the vaporized atoms are excited by a discharge plasma. Since each atom excited in the plasma emits light at a wavelength characteristic of the element, it is possible to determine the quantity of the element by dispersing the emitted light into a spectrum and measuring the light intensity at the aforementioned wavelength. It is also possible to perform a qualitative analysis of an unknown element contained in the sample by creating an emission spectrum with a predetermined wavelength range and searching for a wavelength at which a line spectrum is present. Normally, the spark discharge is repeated at a frequency from a few tens to several hundreds of hertz, and the photometrical values obtained for each discharge are integrated to improve the measurement accuracy.
In this type of emission analyzer, it is necessary to charge the capacitor to a voltage level of several hundreds of volts within a relatively short period of time to generate a spark discharge. For this purpose, a switching type capacitor-discharging circuit has been widely used in recent years (for example, refer to Patent Document 1). FIG. 3 is a block diagram showing the configuration of an emission analyzer using a conventional switching type capacitor-charging circuit.
In this emission analyzer, the emitting section consists of a capacitor-charging circuit 1, a capacitor circuit 2, an igniter circuit 3, and an emission stand 4. The capacitor-charging circuit 1 includes a direct-current (DC) power source 11, a flyback transformer 12 with primary and secondary windings, a switching element 13 such as a field-effect transistor (FET), and a charge controller 14 for driving the switching element 13. The capacitor circuit includes a rectifying diode 21, a discharge capacitor 22 for storing electrical energy to be used for generating a discharge, and a charged voltage detector 23 for detecting the charged voltage of the discharge capacitor 22. The igniter circuit 3 includes an igniter transformer 31 with primary and secondary windings, and an igniter driver 32 for generating a high level of voltage in the secondary winding of the igniter transformer 31. The emission stand 4 includes a discharge electrode 41 and a sample 42 to be measured, which is typically a piece of metal.
In the capacitor-charging circuit 1, the primary winding of the flyback transformer 12 and the switching element 13 are serially connected to both ends of the DC power source 11, respectively. When the switching element 13 is turned on (i.e. made to be conductive) by the charge controller 14, a DC current is supplied to the primary winding of the flyback transformer 12, whereby an excitation energy is stored in the flyback transformer 12. The charge controller 14 maintains the switching element 13 in the “on” state for a predetermined period of time. Subsequently, when the switching element 13 is turned off, a counter electromotive force arises in the secondary winding of the flyback transformer 12. As a result, the excitation energy that has been accumulated in the flyback transformer 12 is supplied through the rectifying diode 21 into the discharge capacitor 22 in the capacitor circuit 2. Thus, the discharge capacitor 22 is charged.
The on/off state of the switching element 13 is controlled as shown in FIG. 4. Every time the switching element 13 is turned off, the charged voltage of the discharge capacitor 22 increases in a stepwise manner due to the excitation energy that has been accumulated in the flyback transformer 12. The charged voltage detector 23 monitors the charged voltage of the discharge capacitor 22. Based on this monitored value, the charge controller 14 determines whether or not the charged voltage has exceeded a predetermined level V1. The charge controller 14 repeats the on/off controlling of the switching element 13 until the charged voltage exceeds the predetermined level V1. When the charged voltage has exceeded the predetermined voltage V1, the charge controller 14 stops turning on the switching element 13 to discontinue the charging of the discharge capacitor 22.
After the charging operation is completed in this manner, the igniter driver 32 in the igniter circuit 3 generates a high voltage in the igniter transformer 31, whereupon a spark discharge occurs between the discharge electrode 41 and the metallic sample 42. This makes the surface of the metallic sample 42 locally heated, vaporizing the atoms of an element present on the sample surface. Simultaneously, the energy stored in the discharge capacitor 22 is transferred into the gap between the discharge electrode 41 and the metallic sample 42 to create a plasma, in which the vaporized atoms are excited by electrons. When an atom returns from the excited state to a stable state, it emits light having a wavelength corresponding to the energy difference between the two states. The photometric section 5, which includes a light-dispersing element, photodetector and other components, measures the emitted light having a wavelength characteristic of the element to collect information relating to the elements contained in the metallic sample 42.
As just described, the capacitor-charging circuit 1 based on the conventional switching method accumulates a required amount of electrical energy in the discharge capacitor 22 by discontinuing the charging operation when the charged voltage of the discharge capacitor 22 has been found to be higher than the predetermined level V1.
However, the previously described capacitor-charging circuit 1 has the following problem: Since the period of time during which the switching element 13 is in the “on” state is definitely set, the amount of excitation energy that is accumulated within each on/off cycle of the switching element 13 changes if the voltage of the DC power source 11 changes. Furthermore, the capacitance of the discharge capacitor 22 varies due to, for example, a temperature change of the capacitor. Such a change in the amount of excitation energy accumulated in the flyback transformer 12 or the capacitance of the discharge capacitor 22 leads to a change in the number of on/off operations of the switching element 13 necessary for charging the discharge capacitor 22 to the constant voltage V1. Then, as shown in FIG. 5, the point in time at which the charged voltage of the discharge capacitor 22 exceeds the threshold level V1 will vary, which means that the charged voltage of the discharge capacitor 22 at the moment of discontinuing the charging operation can change by up to an amount corresponding to one on/off cycle of the switching element 13. Thus, the amount of energy to be accumulated in the discharge capacitor 22 changes.
Even if the charged voltage eventually reaches the same level, if the capacitance of the discharge capacitor 22 changes due to the aforementioned reason, the amount of energy held in the discharge capacitor 22 will change since the discontinuation of the charging operation is controlled based on the monitored value of the charged voltage of the same capacitor 22.
If the energy stored in the discharge capacitor 22 at the moment of generating the spark discharge is not constant, the state of the plasma at the moment of discharging will change. Therefore, even if the element content of the metallic sample 42 is the same, the emission intensity will vary, which may possibly deteriorate the accuracy or reproducibility of the analysis.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2004-333323 (Paragraphs 0004-0007; FIG. 5)